1. Field
The present disclosure relates generally to telecommunications, and more specifically, to pilot assisted channel estimation techniques.
2. Background
In a typical telecommunications system, the data to be transmitted is encoded with a turbo code, which generates a sequence of symbols, referred to as “code symbols.” Several code symbols may be blocked together and mapped to a point on a signal constellation, thereby generating a sequence of complex “modulation symbols.” This sequence may be applied to a modulator, which generates a continuous time signal, which is transmitted over a wireless channel.
At the receiver, the demodulator generates a sequence of soft decisions. Each soft decision represents an estimate of a modulation symbol that was transmitted over the channel. The estimates may be used to compute the Log-Likelihood Ratio (LLR) of the code symbols. The turbo decoder uses the sequence of code symbol LLRs in order to decode the data that was originally transmitted.
When computing the LLRs of the code symbols, the propagation conditions of the channel should be considered. The channel conditions, or the channel impulse response, may be estimated at the receiver from a known pilot sequence embedded in the data transmission. By way of example, in Orthogonal Frequency Division Multiplexing (OFDM) systems, a Least Squares (LS) procedure is often used to estimate the channel. Using this procedure, the channel may be estimated from a set of pilot tones equally spaced across the frequency band ½T≦f≦½T, provided the time interval of the channel's impulse response LT is less than PT, where L is the delay spread in chips between arriving signals, T is the chip duration (time), LT is the time delay, where P is the number of pilot tones and T is the chip duration, and PT is the pilot time duration. Moreover, it can be shown that the channel estimation variance or error is proportional to L and inversely proportional to P.
Assuming equal noise power across the frequency tones, the channel estimation variance can be represented by the following equation:
                              σ          e          2                =                              L            P                    ⁢                      σ            2                                              (        1        )            where σe2 denotes the variance of the channel estimate, and σ2 denotes noise variance per tone.
Typically, a channel estimator in the receiver has a fixed delay spread LT, where L=P. However, this may lead to an unnecessarily large channel estimation variance when the actual impulse response of the channel is small. The channel estimation variance could be improved if the delay spread LT at the receiver is adapted in accordance with the time varying nature of the channel as seen by the receiver.